He told a jury at Isleworth crown court in west London that the race was a symbol of elitism in government.

Judge Anne Molyneux said all options were open to the court, including jail, when he is sentenced on 19 October. "Mr Oldfield has accepted that he disrupted the boat race," she said.

The court heard that Oldfield, an Australian who moved to the UK in 2001, decided to make the protest after learning of government plans to "sell off" the NHS and "snoop" on electronic communications, and after hearing encouragement to "dob in" people planning protests during the Olympics.

Oldfield, of east London, worked and volunteered for a decade in jobs and projects aimed at increasing better prospects for people in impoverished areas.

He stopped the annual contest for around half an hour, the first time in the history of the 158-year event that it had been disrupted by a swimmer.

The prosecutor, Louis Mably, told jurors the race between Oxford and Cambridge was spoiled for hundreds of thousands of spectators watching from the banks of the river or live on BBC TV, not to mention the two university rowing teams.

The judge said it was Oldfield's first offence and that five people had told the court he was a man of good character.

She said the 36-year-old had moved from his native Australia in 2001 and held a number of jobs in social projects. However, she said: "The court will be considering if a custodial sentence is necessary."

Oldfield hugged his partner as he was released on bail until sentencing.

• This article was amended on 27 September 2012 because the original said Trenton Oldfield had moved from Australia in 2011. This has been corrected to say he moved from Australia in 2001.